For achieving a proper cleaning action, a conventional windshield wiper blade includes a rubber element forming a single wiping edge with a cross section defining a hinge extending the length of the element. The hinge is spaced from and extends parallel to the wiping edge. The hinge allows the wiping edge of the element to flip over so that the leading edge of the element tilts to a predetermined angle relative to the glass each time the wiping direction is reversed. The hinge is typically in the form of a reduced thickness section extending the length of the element with spaced apart thicker sections on each side of the element above and below the hinge. Further, the blade typically includes a flexor which strengthens the rubber element above the hinge. The flexor is attached through a superstructure to an arm. The reduced thickness section allows pivotal movement of the wiping edge relative to the more rigidly supported upper portion of the element. As the wiper blade is moved over the windshield, the thicker sections of the rubber element above and below the hinge on the trailing side of the element bump or abut to limit movement of the hinge and thereby establish the optimum wiping angle of the leading edge of the element relative to the glass.
Under adverse conditions, the exposed hinge can become impacted with ice, snow or mud. When the hinge becomes impacted, the edge of the element ceases to flip over to the proper angle relative to the glass when the blade reverses directions. The resulting improper angle between the wiping edge and the glass can result in streaking and/or chatter.
It is known in the windshield wiper art that forming a wiper blade with dual wiping surfaces can have certain advantages For example, U.S. Pat. No. 3,224,027 teaches that wind lift problems can sometimes be solved with a dual wiping edge blade. At high vehicle speeds, wind flowing over the windshield tends to lift the blade from the windshield, thereby preventing effective wiping. If the pressure exerted by a wiper arm on the blade is increased, the wind lift problem can be cured. However, the increased pressure can result in chattering, and depositing of rubber on the windshield which shortens blade life and obscures vision. By adding a second wiping edge to the blade, the contact surface between the blade and the windshield is doubled. Consequently, the overall force exerted on the blade can be increased up to double without increasing the force exerted per unit length of the wiping edge.